1. Field of the Invention
The present invention relates to the field of gas turbine engines and turbojets and pertains more particularly to the arrangement of a spark plug in the combustion chamber of this type of engine.
2. Description of the Related Art
In a gas turbine engine, the combustion chamber receives air from the compressor, a portion of which is mixed with the fuel and burned in the primary combustion zone. Ignition is provided by one or two spark plugs arranged downstream of the carburation system, and another portion of the air circulates around the primary combustion zone and is mixed with the primary combustion gases. All the hot gases are directed toward the turbine. Combustion chambers are designed to meet a number of required specifications, such as in-flight reignition, temperature profile form, pollutant gas emissions and also thermal and mechanical behavior of the various components thereof.
In particular, the ignition system has to guarantee in-flight re-ignition in the event of accidental extinguishing in the combustion chamber, while withstanding the thermal stresses imposed thereon; in particular, it must withstand those stresses. Such conditions involve arrangements that do not sit well together. Indeed, the injection system produces a layer of sprayed fuel that forms a certain angle with the chamber axis. If the latter is very closed, the spark plug is outside of the cone formed by the fuel; from the thermal behavior standpoint, this is favorable, but the ignition capabilities of the chamber are reduced. Conversely, an injection system in which the layer of fuel forms a very open cone gives rise to significant heating-up of that zone of the chamber that surrounds the spark plug owing to the impact of fuel on the walls and the spark plug. The thermal behavior of these elements is adversely affected thereby. Mounting of the spark plug generally takes these thermal conditions into account.
The present invention relates to ignition systems in which the spark plug is mounted on the combustion chamber casing, for example by means of a component forming an adapter that is itself affixed to the chamber casing. The spark plug, downstream of the fuel injection system, extends from the casing radially toward the inside of the chamber, and the end thereof is flush with the inner face of the chamber wall, via an aperture made in said wall.
During operation, lateral clearance is provided around the spark plug so as to allow the relative movements between the chamber and the casing that are the result of variations in temperature, pressures and stresses during different phases of a flight, without the spark plug, which is secured to the casing, abutting against or bearing on the edges of the aperture made in the chamber wall. The wall, at the opening, is provided with a cylindrical component that is reasonably high and forms a well into which the distal end of the spark plug is slipped, and a floating bushing forms a spark-plug guide. The spark-plug guide surrounds the spark plug such as to close the well and to guarantee the seal between the chamber and the zone outside of the chamber, between the chamber and the casing, and an example of this type of spark-plug mounting in a gas turbine engine combustion chamber is shown in the Applicant's patent application FR 2 926 329.
In addition to taking up the relative movements between the chamber and the casing, the well comprises ventilation openings for cooling the spark plug. Air enters from the outside of the combustion chamber, owing to the pressure difference, and maintains the temperature of the end of the spark plug, which is exposed to combustion gases and to the radiation thereof, at an acceptable level owing to the material of which it is composed. Insofar as the ventilation openings are made radially in the well wall, fine air jets form in the direction of the spark-plug surface. The cooling produced by the jets depends on various parameters, including the diameter of the openings and the distance from the latter to the jet impact surface.
However, cooling, which is optimum when the spark plug is at the center of the well, with all the jets traveling the same distance, may be of reduced efficiency when the spark plug is no longer in the center owing to variations in load between the chamber and the casing. Thermal transfers between the air jets and the impact wall, which are calibrated as rated, increase when the jet distance is shorter, on the one hand, and decrease, on the other, when the jet distance is longer than the predefined optimum distance.